The present invention relates generally to the field of light emitting semiconductor devices. More specifically, it relates to transparent substrate ("TS") light emitting diodes ("LED"s). Non-absorbing distributed Bragg reflectors ("DBR"s) are employed to improve the light extraction and optics of a TS LED. The resulting devices exhibit increased flux and intensity compared to known TS LEDs.
A packaged LED is fabricated from a variety of materials, each with a different index of refraction. The index of refraction is usually designated by n, and n varies from .about.3.5 in the LED semiconductor chip to .about.1.5 in the epoxy used to encapsulate the LED chip. This large difference in refractive indices results in a critical angle for total internal reflection of 25.degree. from the LED the epoxy, as given by Snell's Law [.theta..sub.c =sin.sup.-1 (n1/n2)]. This relatively small critical angle for total internal reflection, combined with internal light absorption within the LED chip result in the LED's external quantum efficiency being substantially less than its internal quantum efficiency. The ratio of these quantities, external/internal quantum efficiency, is defined as the LED's extraction efficiency.
The extraction efficiency of an LED chip is most strongly influenced by the structure of the chip. A variety of LED chip structures and the corresponding escape cones of light defined by their critical angles are depicted schematically in FIGS. 1a, 1b, 1c, and 1d. For each of these figures, the cone model excludes multiple pass light extraction, internal absorption, and randomization effects. For absorbing substrate ("AS") LED chips, the number of escape cones is strongly affected by the thickness of the transparent window layer. An AS LED with a thin transparent window layer (&lt;10 .mu.m), shown in FIG. 1a, possesses only a single top escape cone. If the thickness of the window layer is increased to &gt;40 .mu.m, as shown in FIG. 1b, the number of cones increases to three as a result of contributions from the sides of the chip. A DBR mirror, a multiple layer stack of high and low index semiconductor material layers, can be placed between the light emitting active region and the substrate to minimize the substrate's light absorption. However, DBRs only reflect light of near-normal incidence. In an LED, light is emitted isotropically from the active region. Consequently, light of all angles is incident upon the DBR mirror, and only a portion of it is reflected. The remainder passes into the absorbing substrate. In a typical LED with a DBR, only light differing by less than 15.degree.-25.degree. from normal is reflected. Only a portion of the bottom escape cone is reflected by the DBR. Known LED structures with a DBR, shown in FIG. 1c, possess thin transparent windows, which result in a maximum of two escape cones.
The best known structure for light extraction, shown in FIG. 1d, is a TS LED wherein 6 escape cones are possible. Such TS LED chips 10, shown assembled into a finished LED lamp in FIG. 2, are typically mounted with reflective Ag-loaded epoxy 12 in a reflective mold cup 14. A portion of the bottom light cone in this case is captured when it reflects off the Ag-loaded epoxy at the chip's back surface.
The previous discussion did not include the effects of multiple reflection events or randomization of light within the LED. In chips with thick transparent window regions, especially TS LEDs, photons may make multiple passes to the semiconductor surface without being absorbed, increasing the probability that these photons can escape. Randomization of the direction of internally reflected light may occur as a result of scattering at the chip surface or within the chip, allowing more light to escape than that predicted by the single pass models of FIGS. 1a, 1b, 1c, and 1d. These effects can be significant. For TS AlGaAs LEDs, the maximum extraction efficiency is calculated to be 0.24, ignoring the effects of randomization. The extraction efficiency has been experimentally estimated to be 0.30, and the difference between the two values can be attributed to light randomization and extraction of multiple pass light from the LED chip.
Shadowing from absorbing contact layers/metallizations can affect the extraction efficiency. The metal/semiconductor alloy found in alloyed contacts between the metal contact and the semiconductor is highly absorbing. Non-alloyed contacts typically require the presence of a very heavily doped absorbing semiconductor layer adjacent to the contact. Most methods for forming contacts result in absorption of the light over the entire contact area. Various methods for avoiding such absorption have been proposed, including utilizing transparent contacts of indium-tin-oxide ("ITO"). These have generally not been implemented in commercial LEDs as a result of problems with contact resistance, manufacturability, or reliability.
After the light escapes from the chip, it must be focused into a usable radiation pattern. Typically, LEDs are packaged in a polymer, usually epoxy, which is shaped into a lens. The desired radiation pattern is obtained by the shape of the reflector cup and the epoxy lens. Limitations imposed by this relatively simple optical system make it difficult to focus light emitted from the edges or sides of the chip into the center of the radiation pattern. This is especially true for narrow viewing angle lamps whose radiation patterns possess a full-angle at half power ("FAHP")&lt;15.degree.. Light from the top of the chip is relatively easy to focus. For chips emitting the same flux, a chip with predominantly top surface emission will have a higher peak intensity in lamp form, especially for narrow viewing angle lamps, than a chip wherein a substantial amount of light is emitted from the chip's edges or sides.